Compact digital cameras and digital cameras installed in portable telephones and smartphones can now photograph high resolution images that exceed several million pixels, and have begun to be frequently used not only for photographing landscapes and portraits but also for photographing paper documents, memorandum and business cards.
In the case of an iPhone® from Apple Inc. (registered trademark is used based on Aiphone Co. Ltd. licensing), several types of applications called “scanner apps” are known, where a surface image of paper media photographed by an integrated camera is read, edges of the paper media are recognized by the change of continuous pixels, and edges which are in a trapezoidal state because of being photographed diagonally are corrected to be square (keystone correction), so that the image is finished as if the paper media were read by a contact type document scanner.
This technology for correcting an image of a document photographed by a camera is disclosed in Patent document 1. The technology disclosed in Patent document 1, as Claim 7 and paragraph [0047] state, is that red lines RL are created as markers along both sides of a keyboard of a personal computer (main unit PCB), and keystone correction is performed using a tilt angle of the red line with respect to the side edges of the screen of the captured image as a correction parameter.    [Patent document 1] Japanese Patent Application Laid-Open No. 2010-130181